This invention relates generally to a method of manufacture of multilayer printed circuit boards and, more particularly, to the manner in which the ground and voltage planes are formed in such method.
Multilayer printed circuit boards, and various methods of their manufacture, are well known in the electronics industry. Typically, each layer of the printed circuit board consists of an insulative material with a foil (such as copper) on its upper and lower surfaces. A process including the steps of printing and etching may be used to produce a configuration of printed circuit wiring on each side of the layer. Alternatively, a masking and plating process can be used to deposit the printed circuit wiring on the insulative layer. The various layers are then bonded together to form a multilayer printed circuit board by first coating each layer with a prepreg (usually a B-stage epoxy), aligning and stacking the layers, and subjecting the layers to conditions of temperature and pressure which result in the formation of a permanent bond between the prepreg and the various layers. After this lamination process, through holes may be made and subsequently plated.
It is often desirable to provide voltage and ground planes in a multilayer printed circuit board. These planes consist of intermediate metal foils or layers within the multilayer structure. Such layers are typically 2 oz. copper (0.0028 inch), the same thickness as the printed circuit wiring on the various layers of the multilayer board. It is necessary to form clearance holes in the areas of the ground and voltage planes where it is not desired to have a connection between the planes and the plated-through holes. The clearance holes are substantially larger than the plated-through holes to assure that there is no undesirable electrical contact made between the platedthrough hole and the ground or voltage plane. With typical 2 oz. ground and voltage planes in a multilayer board, it is normally the practice to form the clearance holes by etching. For thin metal layers, etching has normally been an acceptable technique for providing an effective clearance area preventing short circuits in the board from the plated-through hole to the clearance hole in the ground or voltage planes.
Recently a demand has arisen for multilayer printed circuit boards capable of distributing high currents, such as 300-400 amps. The typical 2 oz. ground and voltage planes utilized in standard multilayer boards are inadequate for carrying such magnitudes of current. As a consequence, it has become necessary to increase the thickness of the ground and voltage planes. Increasing the thickness of these planes presents a serious problem in the forming of the clearance holes by etching techniques because of under-etched conditions that are created. That is, etching of relatively thick ground and voltage planes to form clearance holes is not reliable for eliminating the problem of undesired electrical contact between the planes and the plated-through holes.
It is the object of the invention to overcome the limitations in present technique of manufacturing multilayer printed circuit boards having relatively thick ground and voltage planes by providing a method which not only eliminates under-etch problems but also is relatively simple and inexpensive to perform.